In recent years, laser projectors which employ laser beams for projection lights have been developed as projector devices. A reflection-type laser projector is disclosed in WO96/038757.
Hereinafter, such a prior art laser projector will be described.
FIG. 14(a) illustrates a structure of a prior art laser projector.
A prior art laser projector 600 has a laser projection unit 40 which outputs laser beams 41, and a screen 610 onto which the laser beams 41 from the laser projection unit 40 are projected. The screen 610 comprises a reflector 611 which reflects incident light such as the laser beams 41, and a diffuser 612 which is placed at the front of the reflector 611 and diffuses light. In this case, ground glass or a group of minute planar lenses having shallow concavo-convexes are used as the diffuser 612.
The laser projection unit 40 has a red laser 1, a blue laser 2, and a green laser 3 as short-wavelength laser sources which emit laser beams of three colors of red, blue, and green. The laser projection unit 40 has corresponding mirrors 51a, 52a, and 53a which reflect the three color laser beams P1, P2, and P3 from the short-wavelength laser sources, and a light modulation unit 20 which modulates the three color laser beams P1-P3, respectively.
FIG. 14(b) is a diagram illustrating a specific structure of the light modulation unit 20.
The light modulation unit 20 has liquid crystal cells 71, 72, and 73 that modulate the lights from the corresponding lasers 1, 2, and 3 in accordance with a video signal Sv, mirrors 51b, 52b, and 53b that reflect the laser beams P1, P2, and P3 which are reflected by the mirrors 51a, 52a, and 53a, respectively, and lens systems 61a, 62a, and 63a which project the laser beams P1-P3 which are reflected by the respective mirrors 51b, 52b, and 53b onto the corresponding liquid crystal cells 71, 72, and 73. In FIG. 14(a), the liquid crystal cells 71, 72, and 73 corresponding to the laser beams P1-P3 are abbreviated as liquid crystal cell 7, respectively, and the lens systems 61a, 62a, and 63a corresponding to the laser beams P1-P3 are abbreviated as lens system 6a, respectively.
The light modulation unit 20 further includes an optical device 8 that outputs the laser beams P1-P3 which are modulated by the liquid crystal cells 71, 72, and 73 with aligning the optical axes thereof with each other, and a lens system 6b that irradiates the laser beams which are outputted from the optical device 8 onto the screen 610 with enlarging those laser beams.
In this case, the red laser 1 outputs an output light of a semiconductor laser as a red laser beam, and the blue laser 2 and the green laser 3 output a blue laser beam and a green laser beam, respectively, utilizing wavelength conversion of the semiconductor laser light. As the screen 610, a screen of gain 1 (a size of 90 inches) that is usually employed in a normal projector utilizing a mercury vapor lamp is employed.
Next, the operation will be described.
In this laser projector 600, the laser beams P1, P2, and P3 that are emitted from the respective lasers 1, 2, and 3 are projected onto the liquid crystal cell 7 through mirrors, and the laser beams that are modulated by the liquid crystal cell 7 are projected onto the screen 610.
More specifically, the red laser 1 exercises a continuous light emitting operation, and the red laser beam P1 emitted therefrom is reflected by the mirrors 51a and 51b to change the destination. Then, the red laser beam P1 reflected by the mirrors 51a and 51b is projected by the lens system 61a onto the liquid crystal cell 71 and is modulated by the liquid crystal cell 71 according to a video signal Sv. The red laser beam P1 modulated by the liquid crystal cell 71 is inputted to the optical device 8, and the red laser beam P1 that is outputted from the optical device 8 is enlarged by the lens system 6b to be projected on to the screen 610. Similarly, the blue laser beam P2 and the green laser beam P3 that are emitted from the blue laser 2 and the green laser 3, respectively, are projected onto the corresponding liquid crystal cells 72 and 73 through mirrors 52a, 53a, 52b, and 53b and lens systems 62a and 63a, and the blue laser beam P2 and the green laser beam P3 which are modulated by the corresponding liquid crystal cells are projected onto the screen 610 through the optical device 8 and the lens system 6b. 
Reflectance of the conventional screen 610 onto which the laser beams are projected is kept approximately constant over a wavelength range except for a wavelength range where luminosity factor, i.e., sensitivity to light of human eyes, is significantly low (i.e., shorter than 400 nm or longer than 700 nm), as shown in FIG. 16.
With this screen 610, a person observes the light that is reflected or scattered by the screen 610 from the front face of the screen 610 (from the side of the laser projection unit). When the entire surface of the screen 610 is full white, the screen 610 has the brightness of approximately 200 lux.
However, in cases where the laser beam 41 is projected on the screen 610 and a person observes the laser beam 41 that is reflected by the screen from the front face of the screen 610, as described above, a phenomenon of “grayish-block” occurs, i.e., a phenomenon in which pictures on the screen 610, which should be originally black, are seen as whitish when for example indoor illumination 30 or outdoor light 31 is applied to the screen 610 as shown in FIG. 15. In FIG. 15, reference numeral 32 denotes a reflected illumination light, and numeral 42 denotes a reflected laser beam.
To be more specific, in a situation where the brightness on the screen 610 is 20 lux when the indoor illumination 30 is on, the contrast of pictures on the screen 610 is 1000:1 when the indoor illumination 30 is turned off, while the contrast is lowered to 10:1 by turning on the indoor illumination 30.
The present invention is made to solve the above-mentioned problem, and it has for its object to provide a laser projector that can prevent pictures on the screen from becoming hard to be seen due to effects of indoor illumination or light from the outdoors.